1. Field of the Invention
This invention relates to a matrix display device comprising a plurality of display elements arranged in an array, each of which comprises electrodes carried on opposing substrates with electro-optical material therebetween and is associated with switching means operable to control the application of data signals to the display element in response to switching signals being applied thereto.
A display device of this kind is suitable for displaying alpha-numeric and video information using passive electro-optical material such as liquid crystal material, electrophoretic suspensions and electrochromic materials.
In known examples of this kind of display device, the display elements are arranged in a matrix of rows and columns and are defined by respective driving electrodes on one of the substrates and opposing portions of a common electrode on the other substrate. A switching means in the form of a transistor, for example a thin film transistor, (TFT), is located adjacent the driving electrode of its respective display element on the one substrate with its drain electrode connected to the driving electrode. The source electrodes of all transistors in the same column are connected to a respective one of a set of column conductors to which data signals are applied and the gate electrodes of all transistors in the same row are connected to a respective one of a set of row conductors to which switching (gating) signals are applied to switch the transistors on. The device is driven by repetitively scanning the row conductors one at a time in sequential fashion so as to turn on all transistors in each row in turn and applying data signals to the column conductors appropriately in synchronism for each row in turn so as to build up a display. When the transistors are on, the data signals are supplied to the associated driving electrodes thus charging up the display elements. When the transistors are turned off, upon termination of the switching voltage, the charge is stored in the display elements concerned until the next time they are addressed with a scanning signal, usually, in the case of a video display, in the next field period.
Display devices of this type are generally well known. Such an active matrix addressed liquid crystal display device may typically consist of 200,000 or more display elements and be capable of displaying TV pictures. The transistors may be bulk transistors, e.g. MOSFETs, formed on a semiconductor substrate of the device. For large area display devices, TFTs deposited on a glass substrate are normally used. With the increasingly larger display areas now being proposed comes a corresponding increase in the number of display elements, and hence switching means, required.
A major problem in making large area display devices of this kind is yield. When using, for example, TFTs deposited on a glass substrate with their associated row and column conductors, just a few defective TFTs can render the device unacceptable. Depending on the nature of the defect, even one defective TFT can lead to complete rows or columns of display elements being unusable. In an attempt to overcome this problem, redundance schemes have been proposed. More particularly, it is known to provide two TFTs for each display element connected in parallel with one another with their gates connected to respective, different, row conductors. If one TFT should prove to be defective after fabrication of the array, it is disconnected from the display element address circuit, for example, by laser scribing, leaving the other TFT to perform the necessary switching function. This technique has its disadvantages however in that it can be time consuming and expensive, and may not always be accomplished satisfactorily. It is necessary to measure each display element to identify and then correct any faults, usually before completing the construction of the device.